Solution-phase inclusion of silver into chalcogenide semiconductor inks

ABSTRACT

Silver-containing absorbers for photovoltaic devices and techniques for fabrication thereof are provided. In one aspect, a method of forming an ink includes: mixing a silver halide and a solvent to form a first solution; mixing a metal, sulfur, and the solvent to form a second solution; combining the first solution and the second solution to form a precursor solution; and adding constituent components for an absorber material to the precursor solution to form the ink. Methods of forming an absorber film, a photovoltaic device, and the resulting photovoltaic device are also provided.

STATEMENT OF GOVERNMENT RIGHTS

This invention was made with Government support under Contract numberDE-EE0006334 awarded by Department of Energy. The Government has certainrights in this invention.

FIELD OF THE INVENTION

The present invention relates to photovoltaic devices and moreparticularly, to silver (Ag)-containing absorbers for photovoltaicdevices and techniques for fabrication thereof.

BACKGROUND OF THE INVENTION

Thin film solar cell absorbers composed of earth-abundant elements suchas Cu₂ZnSn(S,Se)₄ (CZT(S,Se)) are particularly relevant due to theirrelatively low toxicity and their record maximum power conversionefficiency of 12.6%. Despite promising results, further work is howeverneeded to understand how to improve this technology and enable itscommercial-scale implementation.

Recent efforts have identified the need to remove defect states bydeliberate passivation or the introduction of dopants in order toimprove upon the voltage deficits (compared to theoretical limits)exhibited by CZT(S,Se). The inclusion of silver could reduce tail statesthat are introduced by the disorder caused in the random alternation ofcopper and zinc in the kesterite lattice (the copper cation is only 5%larger than the zinc cation). The inclusion of silver has been shown toreduce antisite defects in CZT(S,Se) absorbers by approximately an orderof magnitude.

One process to incorporate silver in CZT(S,Se) involves evaporatingsilver after the deposition of the absorber. This technique can howevercompromise absorber quality by exposing it to air.

An alternative method involves nanoparticle synthesis. See, for example,Wei et al., “Synthesis and Characterization of Nanostructured StanniteCu₂ZnSnSe₄ and Ag₂ZnSnSe₄ for Thermoelectric Applications,” ACS Appl.Mater. Interfaces, April 2015, 7, 9752-9757. However, this synthesisprocedure requires extensive processing and multiple purification stepswhich in turn lead to extremely low yields of nanoparticles. Further,the native ligands that surround the particles are insulating and thusyield low quality devices.

Accordingly, improved techniques for incorporating silver into absorbermaterials like CZT(S,Se) would be desirable.

SUMMARY OF THE INVENTION

The present invention provides silver (Ag)-containing absorbers forphotovoltaic devices and techniques for fabrication thereof. In oneaspect of the invention, a method of forming an ink is provided. Themethod includes: mixing a silver halide and a solvent to form a firstsolution; mixing a metal, sulfur, and the solvent to form a secondsolution; combining the first solution and the second solution to form aprecursor solution; and adding constituent components for an absorbermaterial to the precursor solution to form the ink.

In another aspect of the invention, a method of forming an absorber filmis provided. The method includes: forming an ink by: i) mixing a silverhalide and a solvent to form a first solution, ii) mixing a metal,sulfur, and the solvent to form a second solution, iii) combining thefirst solution and the second solution to form a precursor solution, iv)adding constituent components for an absorber material to the precursorsolution to form the ink; depositing the ink onto a substrate to formthe absorber film on the substrate; and annealing the absorber film.

In yet another aspect of the invention, a method of forming aphotovoltaic device is provided. The method includes: forming anelectrically conductive layer on a substrate; forming an absorber layeron the electrically conductive layer by: i) mixing a silver halide and asolvent to form a first solution, ii) mixing a metal, sulfur, and thesolvent to form a second solution, iii) combining the first solution andthe second solution to form precursor solution, iv) adding constituentcomponents for an absorber material to the precursor solution to form anink, v) depositing the ink onto the electrically conductive layer toform the absorber layer on the electrically conductive layer; annealingthe absorber layer; forming a buffer layer on the absorber layer;forming a transparent front contact on the buffer layer; and forming ametal grid on the transparent front contact.

In still yet another aspect of the invention, a photovoltaic device isprovided. The photovoltaic device includes: a substrate; an electricallyconductive layer on the substrate; an absorber layer on the electricallyconductive layer, wherein the absorber layer comprises silver and ahalide, and wherein the silver and the halide are both presentthroughout the absorber layer; a buffer layer on the absorber layer; atransparent front contact on the buffer layer; and a metal grid on thetransparent front contact.

A more complete understanding of the present invention, as well asfurther features and advantages of the present invention, will beobtained by reference to the following detailed description anddrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an exemplary methodology for forming anink according to an embodiment of the present invention;

FIG. 2 is a diagram illustrating an exemplary methodology for forming anabsorber film according to an embodiment of the present invention;

FIG. 3 is a cross-sectional diagram illustrating a substrate and anelectrically conductive layer on the substrate according to anembodiment of the present invention;

FIG. 4 is a cross-sectional diagram illustrating an absorber layerhaving been formed on the electrically conductive layer according to anembodiment of the present invention;

FIG. 5 is a cross-sectional diagram illustrating a buffer layer havingbeen formed on the absorber layer according to an embodiment of thepresent invention;

FIG. 6 is a cross-sectional diagram illustrating a transparent frontcontact having been formed on the buffer layer according to anembodiment of the present invention;

FIG. 7 is a cross-sectional diagram illustrating a metal grid havingbeen formed on the transparent front contact according to an embodimentof the present invention;

FIG. 8 is a diagram illustrating the composition of a silver(Ag)-containing CZT(S,Se) film prepared in accordance with the presenttechniques according to an embodiment of the present invention;

FIG. 9 is a diagram illustrating the optoelectronic properties ofabsorber films (with varying amounts of Ag) prepared in accordance withthe present techniques according to an embodiment of the presentinvention; and

FIG. 10 is a diagram illustrating characteristics of photovoltaicdevices prepared according to the present techniques with varyingamounts of Ag as compared to a standard device as a control according toan embodiment of the present invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Provided herein are solution-based techniques for incorporating silver(Ag) directly into the precursor solutions of CIG(S,Se), CZT(S,Se)and/or CIS materials. Advantageously, the present techniques allow theinclusion of Ag in solution without having to expose the material toair. Further, the solution processing of thin film solar absorbers canenable the commercial scale implementation of materials like theearth-abundant, non-toxic CZT(S,Se) absorber.

As its name implies, CZT(S,Se) is a kesterite absorber materialcontaining copper (Cu), zinc (Zn), tin (Sn), and at least one of sulfur(S) and selenium (Se). For a general discussion on kesterites and use ofkesterite in solar cells, see, for example, Mitzi et al., “Prospects andperformance limitations for Cu—Zn—Sn—S—Se photovoltaic technology,” PhilTrans R Soc A 371 (July 2013), the contents of which are incorporated byreference as if fully set forth herein.

A CIG(S,Se) absorber material contains Cu, indium (In), gallium (Ga),and at least one of S and Se. A CIS absorber material includes Cu, In,and Se. The term “absorber” refers to the use of these materials as theabsorber layer in photovoltaic devices. An exemplary photovoltaic devicehaving a CIG(S,Se), CZT(S,Se) and/or CIS absorber layer formed using thepresent techniques will be described below.

A notable challenge to including silver into these materials is thatsilver reacts aggressively with sulfur, instantly forming silver sulfide(Ag₂S), which is undesirable. Advantageously, it has been found hereinthat if one first coordinates the sulfur with a metal such as copperthereby making the sulfur less reactive, then the Ag can be introducedwithout Ag₂S formation. While copper is a suitable choice for any of theabove absorber materials, any metal can be employed for coordination ofthe sulfur.

In general, the present techniques involve separately preparing mixturesof i) a silver halide such as silver chloride (AgCl) silver bromide(AgBr) or silver iodide (AgI) and ii) Cu and S in a solvent such ashydrazine or thiol-amine. After the constituent components havedissolved, the mixtures (i) and (ii) are then combined. This precursorsolution can then be integrated with other constituent (e.g., CIG(S,Se),CZT(S,Se) and/or CIS) components to form an ink which can then beapplied during fabrication of the photovoltaic device absorber.

The present techniques are now described in detail by way of referenceto methodology 100 of FIG. 1 for forming an ink. In step 102, a firstsolution (Solution 1) is prepared by mixing a silver halide and asolvent. According to an exemplary embodiment, the silver halide is AgX,wherein X═Cl, Br, or I. Suitable solvents include, but are not limitedto, hydrazine, and thiol-amine solvents such as 11-Amino-1-undecanethiolhydrochloride.

According to an exemplary embodiment, the solvent is first cooled to atemperature of from about −5° C. to about −2° C., and rangestherebetween. The silver halide salt is then added to the cooledsolvent, and the solution is mixed using, e.g., 30 minutes of vigorousstirring, until the silver halide salt is uniformly dispersed in thesolvent.

In step 104, in a separate vial a second solution (Solution 2) isprepared by mixing a metal, sulfur, and the solvent. Again the solvent(e.g., hydrazine or thiol-amine) is first cooled to a temperature offrom about −5° C. to about −2° C., and ranges therebetween. The metaland sulfur are then added to the cooled solvent, and the solution ismixed using, e.g., 30 minutes of vigorous stirring, until the metal andsulfur have uniformly dispersed in the solvent. As described above, themetal will coordinate the sulfur, making it less reactive when thesilver is later added (see below). According to an exemplary embodiment,the metal is copper. Copper is a preferred choice since the resultingprecursor solution can then be used to form any of the above absorbermaterials. However, any other suitable metals may be employed, such as,indium (In), gallium (Ga), zinc (Zn), tin (Sn), etc.

By way of example only, the following formulation may be used to createa 1% Ag-containing CZT(S,Se) ink:

-   -   Cu 676 milligrams (mg)    -   AgI 25 mg    -   Sn 714 mg    -   S 504 mg    -   zinc formate 1.070 grams (g)    -   Se 3.00 g,        wherein 4 milliliters (mL) of hydrazine are used to        dissolve/coordinate Cu₂S, and 8 mL of hydrazine is used for the        rest of the material. In this example, the Cu is used to        coordinate the S, and however much Ag is added is subtracted        from the amount of Cu. Thus, if more Ag is added to the        formulation, then the amount of Cu is reduced. As described        below, the 1% Ag devices exhibited efficiencies of 10.5%.

In step 106, Solution 1 and Solution 2 are combined (i.e., mixed) toform a precursor solution. When combined, Solution 1 and Solution 2 willbubble, and it is preferable to wait until the bubbling has stopped,e.g., about 3 minutes, before proceeding to the next step.

In step 108, the precursor solution is used to form an ink with otherconstituent components of the given absorber material. For instance, fora CZT(S,Se) absorber one would add zinc (Zn), tin (Sn), and sulfur (S)and/or selenium (Se). Each element can be weighed separately and addedto the precursor solution. For a CIGS ink, one would add indium (In),gallium (Ga), sulfur (S) and/or selenium (Se), while for a CIS ink onewould add indium (In), and selenium (Se).

The ink can then be used to form a film of the respective absorbermaterial. See, for example, methodology 200 of FIG. 2. For instance, instep 202, the ink is prepared as described above in accordance withmethodology 100. Of course, the constituent components added to theprecursor solution will depend on the desired composition of theabsorber film, e.g., CZT(S,Se), CIGS, CIS, etc. As described above, theprocess involves separately mixing i) the sulfur with a metal (whichmakes the sulfur less reactive) and ii) a silver halide in a suitablesolvent (e.g., hydrazine or thiol-amine). While any of theabove-described metals can be used in this manner to coordinate thesulfur, copper is a good choice since it is the basis for all three ofthe absorber materials.

In step 204, the ink is then deposited (i.e., cast) onto a substrate,forming a film on the substrate. Suitable casting processes include, butare not limited to spray coating, spin coating, ink jet printing, etc.

According to an exemplary embodiment, the present techniques areimplemented in the fabrication of a photovoltaic device where the (e.g.,CZT(S,Se), CIGS, CIS) film serves as the absorber layer of the device.As will be described in detail below, in that case, the substrate can bean electrically conductive substrate, such as a molybdenum (Mo)-coatedglass substrate.

In step 206, the film is annealed. Annealing serves to improve the grainstructure of the film. According to an exemplary embodiment, the annealis performed at a temperature of from about 400 degrees Celsius (° C.)to about 800° C., and ranges therebetween, for a duration of from about100 seconds to about 120 seconds, and ranges therebetween. Preferably,the annealing is performed in an environment containing excess chalcogen(e.g., sulfur (S) and/or selenium (Se)) which serves to replace thesevolatile elements lost during heating.

According to an exemplary embodiment, the present techniques areemployed in the fabrication of a photovoltaic device. This exemplaryembodiment is now described by way of reference to FIGS. 3-7. As shownin FIG. 3, the process begins with a substrate 302 coated with a layer304 (or optionally multiple layers represented generically by layer 304)of an electrically conductive material.

Suitable substrates 302 include, but are not limited to, glass (e.g.,soda lime glass (SLG)), ceramic, metal foil, or plastic substrates.Suitable materials for forming (electrically) conductive layer 304include, but are not limited to, molybdenum (Mo), molybdenum trioxide(MoO₃), gold (Au), nickel (Ni), tantalum (Ta), tungsten (W), aluminum(Al), platinum (Pt), titanium nitride (TiN), silicon nitride (SiN), andcombinations including at least one of the foregoing materials (forexample as an alloy of one or more of these metals or as a stack ofmultiple layers such as MoO₃+Au).

According to an exemplary embodiment, the conductive layer 304 is coatedon substrate 302 to a thickness of greater than about 0.1 micrometers(μm), e.g., from about 0.1 μm to about 2.5 μm, and ranges therebetween.In general, the various layers of the device will be depositedsequentially using a combination of vacuum-based and/or solution-basedapproaches. By way of example only, the electrically conductive material304 can be deposited onto the substrate 302 using evaporation orsputtering.

Next, an absorber layer 402 is formed on the conductive layer 304. Theabsorber layer 402 is formed according to the processes described inconjunction with the description of FIGS. 1 and 2, above. Namely, an inkis prepared in accordance with methodology 100 (of FIG. 1) whichinvolves separately mixing i) the sulfur with a metal (which makes thesulfur less reactive) and ii) a silver halide in a suitable solvent(e.g., hydrazine or thiol-amine). The solutions are then combined into aprecursor solution to which constituent components are added to form anink. The constituent components added will tailor the composition of theabsorber layer 402. For instance, as provided above, for a CZT(S,Se)absorber one would add Zn, Sn, and S and/or Se. Each element can beweighed separately and added to the precursor solution. For a CIGS ink,one would add In, Ga, S and/or Se, while for a CIS ink one would add In,and Se. See, for example, FIG. 4 where it is indicated that the absorberlayer 402 can be a CZT(S,Se), CIGS, or CIS film.

Next, in accordance with methodology 200 (of FIG. 2), the ink is cast(e.g., using spray coating, spin coating, ink jet printing, etc.) ontothe substrate 302/conductive layer 304, forming absorber layer 402 onthe conductive layer 304. According to an exemplary embodiment, theabsorber layer 402 is a film having a thickness of from about 0.5micrometers (μm) to about 2 μm, and ranges therebetween.

Since the as-deposited materials have poor grain structure and a lot ofdefects, following deposition of the absorber layer 402 a post anneal ina chalcogen (e.g., S and/or Se) environment is preferably performed. Ananneal in a chalcogen environment improves the grain structure anddefect landscape in the absorber material. As provided above, suitableconditions for the anneal include a temperature of from about 400degrees ° C. to about 800° C., and ranges therebetween, for a durationof from about 100 seconds to about 120 seconds, and ranges therebetween.

As will be described in detail below, due to the unique nature of thepresent process, the absorber layer 402 will have a unique composition.For instance, the presence of halide (i.e., Cl, Br, or I) in theprecursor solution will translate to the final film composition, whereina relatively uniform composition of the halide can be found throughoutthe absorber film. See below.

As shown in FIG. 5, a buffer layer 502 is then formed on the absorberlayer 402. See FIG. 5. The buffer layer 502 forms a p-n junction withthe absorber layer 402. According to an exemplary embodiment, the bufferlayer 502 is formed having a thickness of from about 100 angstroms (Å)to about 1,000 Å, and ranges therebetween.

Suitable buffer layer materials include, but are not limited to, cadmiumsulfide (CdS), a cadmium-zinc-sulfur material of the formulaCd_(1-x)Zn_(x)S (wherein 0<x≤1), indium sulfide (In₂S₃), zinc oxide,zinc oxysulfide (e.g., a Zn(O,S) or Zn(O,S,OH) material), and/oraluminum oxide (Al₂O₃). According to an exemplary embodiment, the bufferlayer 502 is deposited on the absorber layer 402 using standard chemicalbath deposition.

As shown in FIG. 6, a transparent front contact 602 is then formed onthe buffer layer 502. Suitable transparent front contact materialsinclude, but are not limited to, transparent conductive oxides (TCOs)such as indium-tin-oxide (ITO) and/or aluminum (Al)-doped zinc oxide(ZnO) (AZO). According to an exemplary embodiment, the transparent frontcontact 602 is formed on the buffer layer 502 by sputtering.

Finally, a metal grid 702 is formed on the transparent front contact602. See FIG. 7. Suitable materials for forming the metal grid 702include, but are not limited to, nickel (Ni) and/or aluminum (Al).According to an exemplary embodiment, the metal grid 702 is formed onthe transparent front contact 602 using evaporation or sputtering.Reference will be made below to a front contact and a back contact. Thefront contact is the transparent front contact 602 and the back contactis the electrically conductive layer 304.

Based on the composition of the precursor solution used in the absorberfilm formation (see above) one would expect a commensurate compositionin the final film. For instance, based on the inclusion of a halide salt(e.g., silver chloride (AgCl), silver bromide (AgBr), or silver iodide(AgI)) in the precursor solution, one would expect to see the respectivehalide distributed throughout the film. This is in fact the result. See,for example, FIG. 8.

FIG. 8 is a diagram illustrating the composition of a CZT(S,Se) absorberfilm prepared in accordance with the present techniques. In thisexample, the silver halide employed in the precursor solution is silveriodide (AgI). Depth (measured in micrometers (μm)) from the frontcontact to the back contact is plotted on the x-axis, and Secondary IonMass Spectrometry (SIMS) counts (measured in arbitrary units (AU)) isplotted on the y-axis. As shown in FIG. 8, there is a constant Agintensity (dashed line) throughout the film. Also, while the halidecomposition (in this case iodide (I)) does fluctuate depending on thedepth, the halide is present throughout the entire film. As providedabove, this is a unique result of the present process due to the silverhalide component of the precursor ink solution.

FIG. 9 is a diagram illustrating how the inclusion of small amounts ofAg have a marked impact on the optoelectronic properties of therespective (in this example a CZT(S,Se)) absorber film. In FIG. 9,wavelength (measured in nanometers (nm)) is plotted on the x-axis, andexternal quantum efficiency (EQE (measured in percent (%)) is plotted onthe y-axis. Samples containing 1%, 5%, and 10% Ag show successiveincreases in device efficiency, which is especially apparent atwavelengths from 1100 nm to 1200 nm.

FIG. 10 further illustrates the benefits of the present Ag-containingabsorber films over their 0% Ag counterparts. The films in this examplewere CZT(S,Se) prepared using the present techniques with varyingamounts of Ag (i.e., 1% Ag, 5% Ag, and 10% Ag) and the same film withoutAg (0% Ag) was prepared as a control. As shown in FIG. 10, the samplewith 1% Ag had a power conversion efficiency (PCE) of 10.5%, a fillfactor (FF) of 66.4%, an open circuit voltage (Voc) of 451.3 millivolts(mV), and a short circuit current (Jsc) of 34.9 milliamps per squarecentimeter (mA/cm²).

Although illustrative embodiments of the present invention have beendescribed herein, it is to be understood that the invention is notlimited to those precise embodiments, and that various other changes andmodifications may be made by one skilled in the art without departingfrom the scope of the invention.

What is claimed is:
 1. A method of forming an ink, the methodcomprising: mixing a silver halide and a solvent to form a firstsolution; mixing a metal, sulfur, and the solvent to form a secondsolution; combining the first solution and the second solution to form aprecursor solution; and adding constituent components for an absorbermaterial to the precursor solution to form the ink.
 2. The method ofclaim 1, wherein the silver halide is selected from the group consistingof: silver chloride, silver bromide, and silver iodide.
 3. The method ofclaim 1, wherein the solvent is selected from the group consisting of:hydrazine, and a thiol-amine solvent.
 4. The method of claim 1, whereinthe solvent comprises 11-Amino-1-undecanethiol hydrochloride.
 5. Themethod of claim 1, wherein the metal is selected from the groupconsisting of: copper, indium, gallium, zinc, and tin.
 6. The method ofclaim 1, wherein the metal comprises copper.
 7. The method of claim 1,further comprising: cooling the solvent to a temperature of from about−5° C. to about −2° C., and ranges therebetween prior to forming thefirst and second solutions.
 8. The method of claim 1, wherein theconstituent components for the absorber material comprise zinc, tin, andat least one of sulfur and selenium.
 9. The method of claim 1, whereinthe constituent components for the absorber material comprise indium,gallium, and at least one of sulfur and selenium.
 10. The method ofclaim 1, wherein the constituent components for the absorber materialcomprise indium and selenium.
 11. A method of forming an absorber film,the method comprising: forming an ink by: i) mixing a silver halide anda solvent to form a first solution, ii) mixing a metal, sulfur, and thesolvent to form a second solution, iii) combining the first solution andthe second solution to form a precursor solution, iv) adding constituentcomponents for an absorber material to the precursor solution to formthe ink; depositing the ink onto a substrate to form the absorber filmon the substrate; and annealing the absorber film.
 12. The method ofclaim 11, wherein the silver halide is selected from the groupconsisting of: silver chloride, silver bromide, and silver iodide. 13.The method of claim 11, wherein the metal is selected from the groupconsisting of: copper, indium, gallium, zinc, and tin.
 14. The method ofclaim 11, wherein the constituent components for the absorber materialcomprise zinc, tin, and at least one of sulfur and selenium.
 15. Themethod of claim 11, wherein the constituent components for the absorbermaterial comprise indium, gallium, and at least one of sulfur andselenium.
 16. The method of claim 11, wherein the constituent componentsfor the absorber material comprise indium and selenium.
 17. The methodof claim 11, wherein the absorber film is annealed at a temperature offrom about 400° C. to about 800° C., and ranges therebetween, for aduration of from about 100 seconds to about 120 seconds, and rangestherebetween.
 18. A method of forming a photovoltaic device, the methodcomprising: forming an electrically conductive layer on a substrate;forming an absorber layer on the electrically conductive layer by: i)mixing a silver halide and a solvent to form a first solution, ii)mixing a metal, sulfur, and the solvent to form a second solution, iii)combining the first solution and the second solution to form a precursorsolution, iv) adding constituent components for an absorber material tothe precursor solution to form an ink, v) depositing the ink onto theelectrically conductive layer to form the absorber layer on theelectrically conductive layer; annealing the absorber layer; forming abuffer layer on the absorber layer; forming a transparent front contacton the buffer layer; and forming a metal grid on the transparent frontcontact.
 19. The method of claim 18, wherein the constituent componentsfor the absorber material comprise zinc, tin, and at least one of sulfurand selenium.
 20. The method of claim 18, wherein the constituentcomponents for the absorber material comprise indium, gallium, and atleast one of sulfur and selenium.
 21. The method of claim 18, whereinthe constituent components for the absorber material comprise indium andselenium.